intellectual activities, we have presented to us an endless coil of interrelations and changes in which nothing is constant whatever be its nature, time or position, but every thing is in motion, suffers change, and passes away. This original, naive and very nearly correct philosophy of the world is that of the old Greek philosophers and was first put in a very clear form by Heraclitus. Everything is and yet is not, since everything is in a state of flux, is comprehended as undergoing constant modification, as eternally existing and disappearing. But this philosophy, correct as it is as regards phenomena in general, viewed, as a picture, is insufficient to explain the individual phenomena of which the picture of the universe is composed, and as long as we cannot do that we are not clear about the general picture. In order to study these individual phenomena we are obliged to take them out of their natural or social connection, and examine each of them by itself according to its own form and its particular origin and development. This is the task of natural science and historical investigation, branches of discovery to which the Greeks of classical times assigned a subordinate place for very good reasons, since they, first of all, had to collect the material. The beginning of an exact observation of nature was made first by the Greeks of the Alexandrine period, and was later developed further by the Arabs in the Middle Ages. True natural science hence dates from the second half of the fifteenth century, and from then on has advanced at a constantly growing rate. The dissection of nature into its separate parts, the separation of different natural events and natural conditions into certain classes, the examination of the interiors of organic bodies with respect to their manifold anatomical forms, furnished the fundamental reasons for the progress in a knowledge of
